Field of the Invention
The present invention relates generally to railway networks and control systems used in connection with operating trains in the railway network, and in particular to systems and methods for detecting broken rails in the tracks, especially in railway systems, such as railway systems that implement communications-based train control systems and methods.
Description of Related Art
Conventional train signal systems use track circuits for two basic functions: train detection and broken rail detection. In addition, conventional alternating current (AC) coded track circuits are used for track-to-train communications of signal aspect data. The most common type of track circuit used in non-electrified lines is the direct current (DC) track circuit, which was invented in 1872 and is still widely used today. There are many variations to DC track circuits, including coding to extend lengths and transfer signal information between trackside locations via rails. These variations to DC track circuits use insulated joints to isolate adjacent track circuits, and are typically applied to define signal block sections, which are related to signal locations and fixed block train control systems. The signal block sections are used to maintain a safe separation distance between trains.
Audio frequency (AF) track circuits are commonly used in metro signal applications, where shorter headways are required to support trains with shorter stopping distances. AF track circuits are also applied to electrified lines where DC track circuits do not work. AF track circuits do not require insulated joints, but are limited in length due to rail inductance. More specifically, rail inductance typically limits lengths of AF track circuits to about 1 km, as compared to about a 5 km length limit for DC track circuits. Moreover, AF track circuits are more complex and expensive to build and operate than DC track circuits. The combination of increased cost and length limitations render AF track circuits economically impractical for application to lines designed for non-electrified freight traffic.
Communications Based Train Control (CBTC) systems are based upon trains determining and reporting their locations to a control office via radio data communications. A train may also be equipped to monitor its integrity, e.g., to ensure that the train remains connected together as a single unit with a location of each end of the train being known and reported to the control office. CBTC systems may be applied as a moving block configuration, which maintains safe separation distances between trains based upon communications between each of the trains and an office dispatch system. Train separation distances may thus be reduced by the “moving block” configuration based upon train speeds and braking capabilities. When the “moving block” configuration is combined with newer train braking systems, e.g., electrically-controlled pneumatic (ECP) brakes, braking distances can be further reduced. Safer operation of trains with smaller separation distances therebetween, as well as removal of fixed block and associated wayside signals, can accordingly be supported by CBTC systems.
Conventional CBTC systems can eliminate the need for block track circuits for train detection and associated safe train separation distance functions, but they do not address how to detect broken rail conditions. Conventional track circuits may therefore be applied in addition to the CBTC systems to provide for broken rail protection. The basic configuration of a track circuit is two parallel rails in a series arrangement with an electrical signal transmitter and electrical signal receiver. The rail vehicle wheels and axle spanning the rails in a section of track provide an electrical shunt between the rails. The shunt path created by the railway car causes the transmitted signal to detect the presence of the train in the section of track. The detected presence is used to activate upstream wayside signals to command approaching trains to slow or stop prior to entering an occupied section. Further, certain traditional railroad signaling systems involving track circuits are being replaced in some applications by CBTC technology whereby train position, speed, and direction are communicated via continuous bi-directional communications between vehicles and wayside computers. Examples of CBTCs include the Electronic Train Management System (ETMS) of Wabtec Corporation. While CBTC technology does not require track circuits to detect trains, such circuits may be retained for broken rail protection.
Conventional track circuits come in many different types, but standard signal applications use “normally energized” circuits, which have a power source on one end (for example, a battery) and a receiver (for example, a relay-activated switch) on the other end. When the train shunts the track, it shorts out the circuit and the relay drops. In this manner, the continuous current through the relay coil holds the switch in position indicative of the track section not occupied. An alternate track circuit configuration is “normally de-energized.” The power source and the receiver are at the same end of the section. Power is applied as a train approaches the section. The train shunt completes the circuit and energizes the relay to indicate train presence. This is inherently not “fail-safe,” as failure of the battery or relay could cause the relay to drop. An advantage of the “normally de-energized” track circuit with transmitter and receiver at the same end is the ability to check the track circuit for breaks while the train is within the track section provided the transmit/receive end is ahead of the train. Still further, AC coded track circuits may provide on-board detection of rail breaks when the train is within the section. In this case, the transmitter is on the far side of the section from the receiver with the train approaching the transmitter and while receiving coded signals with pick-up coils ahead of the lead axle. This is considered the safest form of traditional automatic train protection due to the continuous communications of the signal aspect data as well as ability to reflect rail breaks directly ahead of the train within the section (track circuit).
Single track networks typically have passing sidings (or stations) spaced 25 to 30 kilometers apart. Within the sidings/stations, which are typically around 3 kilometers long, and as discussed, broken rail detection may be provided with conventional DC track circuits. Due to low traffic density, there may not be a need for closely following trains in the block sections between sidings/stations. On-board systems, e.g., ETMS, and office systems presently provide train location functions, which eliminates the need for conventional track circuits for the entire network.
Therefore, there is a need in the art for improved broken rail detection systems and methods. There is also a need in the art for long distance broken rail detection systems and methods. With specific reference to light traffic, single track rail networks, there is a need in the art for technology that may be used to support the remote operation of switch machines, without the expense and need for full wayside signal and track circuit system.